The 3rd Generation Partnership Project, 3GPP, is responsible for the standardization of the Universal Mobile Telecommunication System, UMTS, and Long Term Evolution, LTE. The 3GPP work on LTE is also referred to as Evolved Universal Terrestrial Access Network, E-UTRAN. LTE is a technology for realizing high-speed packet-based communication that can reach high data rates both in the downlink and in the uplink and is thought of as a next generation mobile communication system relative to UMTS. In order to support high data rates, LTE allows for a system bandwidth of 20 MHz, or up to 100 MHz when carrier aggregation is employed. LTE is also able to operate in different frequency bands and can operate in at least Frequency Division Duplex, FDD, and Time Division Duplex, TDD, modes.
In an E-UTRAN, a User Equipment, UE, or a wireless device is wirelessly connected to a Radio Base Station, RBS, commonly referred to as a NodeB, NB, in UMTS, and as an evolved NodeB, eNodeB or eNB, in LTE. A Radio Base Station, RBS, or an access point is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE. In Wireless Local Area Network, WLAN, systems the wireless device is also denoted as a Station, STA.
LTE uses downlink reference signals transmitted by the eNodeBs. A user equipment, UE, receiving the reference signal can measure the quality of neighbor cells for mobility purposes. In LTE, the some reference signals are broadcasted in an always-on manner and over the full bandwidth, regardless of the presence or position of UEs in the system. These signals are called cell specific reference signals, CRS, and are easy to measure and yield consistent results, but the static signaling leads to high resource usage, interference, and energy consumption. Hence, the CRS do not depend/change per user but remain same for all the users and entire system, or part of the system, once configured.
In the future communication networks, also referred to as the 5th generation mobile networks, there will be evolvement of the current LTE system to the so called 5G system. The main task for 5G is to improve throughput and capacity compared to LTE. This is in part to be achieved by increasing the sample rate and bandwidth per carrier. 5G is also focusing on use of higher carrier frequencies i.e., above 5-10 GHz.
Future communications networks are expected to use advanced antenna systems to a large extent. With such antennas, signals will be transmitted in narrow transmission beams to increase signal strength in some directions, and/or to reduce interference in other directions. When the antenna is used to increase coverage, handover may be carried out between transmission beams of the serving radio access network node or of the neighbour radio access network nodes. The transmission beam through which the radio access network node is currently communicating with the wireless device is called the serving beam and the transmission beam it will hand over to, or switch to, is called the target beam. The potential target beams for which measurements are needed are called candidate beams.
Applying the principle of continuous transmission of reference signals in all individual transmission beams in such a future cellular communications network may be convenient for wireless device measurements, but it may degrade the performance of the network. For example, continuous transmission of reference signals in all individual transmission beams may consume resources available for data, and generate a lot of interference in neighbouring cells, and higher power consumption of the radio access points.
To avoid always-on signaling, one possible approach is that the network turns on beam quality measurement signals, MRS, in a UE-specific manner only in relevant candidate beams and in situations when mobility is likely needed (e.g. when signal strength is decreasing and/or load balancing needs to be applied). Then the candidate beams are selected from a fixed grid of beams. Measurements are initiated when the network obtains that a beam update for the UE may be needed, e.g. when decreasing serving beam quality is detected due to UE movement, or when the UE needs to acquire a serving beam when accessing a new frequency band for the first time. The candidate beams may be transmitted from a single access point or from several access points. The network can configure the UE (via RRC signaling) to measure and report candidate beam quality, preferably including the list of MRSs to measure. The UE thus receives an MRS measurement command indicating the Time/Frequency, T/F resources and sequences of the MRS to measure, as well as the measurement and reporting configuration. Once the UE has performed mobility measurements and reported the results, the network turns the candidate beams off again, i.e. the MRS transmissions in the candidate beams cease. Separation between MRS of adjacent beams may be achieved e.g. using multiplexing into different time-frequency fields and/or using code multiplexing using (near-) orthogonal sequences here referred to as MRS sequences (or beam quality signal sequences), occupying the same time-frequency field.
The MRS signatures and T/F resource allocation for the individual candidate beams to a given UE is preferably coordinated among the access points transmitting candidate beams e.g. via a central coordination unit and inter-access point interfaces or via an enhanced X2-type interface connecting the 5G eNBs. The coordination is done to guarantee unique MRS sequences and/or efficient use of T/F resources. The coordinated allocation is done on a per-UE and per-session basis to allow high flexibility for resource allocation to beams. Here, a session denotes receiving a measurement command, performing a set of measurements of one or more beams, and reporting the results. The per-session coordination is preferably required. It is not efficient system design, and often not even practical, to associate every beam in every access point with fixed unique MRS parameters—that would imply an excessively large MRS sequence space and reserved T/F resources unavailable for other purposes.
The number of candidate beams that are activated at mobility measurement sessions varies, but anywhere from a few to several tens of beams may be turned on in some scenarios per one UE. The number of UEs in a certain geographical area also varies, but up to 4000 UEs/km2 may be a typical hotspot density.
However, when the UE density is high, when the beams are wide and/or when the UEs in the network are moving at high velocities, or when beams from source and/or target access points support mobility sessions for several UEs, then the network needs to activate a large number of beam quality reference signals from different access points. This may lead to inefficient usage of resources, beam quality reference signal sequences etc. Hence, there is a need for an improved resource usage during for example mobility measurements sessions.